Method for manufacturing a master die for a diffusion plate and diffusion manufactured by said method

ABSTRACT

A diffusion plate includes a plurality of superimposed basic patterns, each having a large number of microstructures, wherein the microstructures are located in two dimensional periodic arrangements, and wherein lattice vectors, corresponding to the periodic arrangement of the microstructures, vary in accordance with the pattern. The invention also discloses a method for manufacturing a master die for such a diffusion plate, and a focusing screen using the diffusion plate.

This application is a division of application Ser. No. 08/498,653, filedJul. 3, 1995, now U.S. Pat. No. 5,733,710, which is a continuation ofapplication Ser. No. 08/411,862, filed Mar. 28, 1995, now abandoned,which is a continuation of application Ser. No. 08/339,295, filed Nov.10, 1994, now abandoned, which is a continuation of application Ser. No.07/985,251, filed Dec. 3, 1992, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a diffusion plate which is used, forexample, as a focusing screen in a single lens reflex camera, and amethod for manufacturing a master die for such a diffusion plate.

2. Description of Related Art

In a known single lens reflex camera or the like, a focusing screen islocated at a position that is optically equivalent to a film plane, sothat a photographer can compose an image having a desired focus byobserving the image through a view finder.

It is known to make the focusing screen using a diffusion plate havingmicroscopic projections and indentations (i.e., an uneven surface) toobserve an unsharp image formed on the focusing screen (i.e., diffusionplate), owing to the characteristic of the diffusion plate to diffuselight.

In a known method to prepare such a diffusion plate, the outer surfaceof a master die is subject to sanding or sand blasting to form microprojections and indentations thereon which are then transferred orcopied onto a plastic optical element of acrylic resin, etc.

The diffusion plate thus prepared has micro projections and indentationsof irregular shape forming a combination of micro prisms having acuteapex angles. Consequently, part of the light incident upon the diffusionplate from a taking lens side is refracted or bent by the apexes atacute angles. Accordingly, much of the light is lost through the viewfinder before reaching the photographer's eye. Furthermore, when adiaphragm is stopped-down, the granularity of the diffusion platebecomes visible, resulting in a poor image quality.

To eliminate the drawbacks mentioned above, it is known to prepare amaster die for a diffusion plate with an uneven surface having a regularmicropattern of smooth apexes using optical means, such as a photoresistprocess, instead of mechanical means, such as sanding sand blasting etc.A molding die is manufactured by copying the master die using anelectroforming process, so that the regular micropattern can betransferred to an optical element (diffusion plate), as disclosed forexample in Japanese Unexamined Patent Publication No. SHO55-90931 orSHO57-148728.

As is well known, the diffusibility is represented by a Fouriertransform of a transmission function of a diffusion plate. Thetransmission function f(x, y) of a conventional diffusion plate having aregular (two-dimensional periodical) pattern is obtained by; ##EQU1##wherein g(x, y) designates the transmission function of amicrostructure, and p=(p_(x), p_(y)) and q=(q_(x), q_(y)) the twodimension periodic lattice vectors, δ the Dirac's δ function, and * *the two dimensional convolution integration, respectively. In thisspecification, p, q, and r, where referred to, represent vectors.

The Fourier transform f(ω_(x), ω_(y)) of f(x, y) is given by thefollowing discrete function. ##EQU2## wherein G(ω_(x), ω_(y)) designatesthe Fourier transform of the function g(x, y), and (a₁, b₁) and (a₂, b₂)the two dimension lattice vectors of the diffusibility.

There is a relationship between (p_(x), p_(y)) and (q_(x), q_(y)) asfollows.

    D=a.sub.1 b.sub.2 -a.sub.2 b.sub.1 =(-p.sub.x q.sub.y -p.sub.y q.sub.x).sup.-1

    a.sub.1 =p.sub.y D, b.sub.1 =-p.sub.x D, a.sub.2 =-q.sub.y D, b.sub.2 =q.sub.x D

ω_(x) and ω_(y) are given by:

    α=λω.sub.x, β=λω.sub.y, γ=(1-α.sup.2 -β.sup.2).sup.1/2

wherein (λ, β, γ) designates the direction cosine in the observationdirection of the diffusion, and λ the wavelength, respectively.

Therefore, the conventional diffusion plate having a regular (twodimension periodical) pattern functions as a diffraction grating.Accordingly, the discrete diffusibility is inevitable. Consequently, anoff-axis aberration, in which multi-lined images appear when a defocused(out of focus) image is viewed, takes place. Furthermore, since thediffraction angle varies depending on the wavelength, if the pitch(i.e., lengths p and q of the two dimension lattice vectors) is smalland the diffraction angle is large, there is a conspicuous irregularityin color of the observed image.

It is possible to prevent the multi-lined image and irregularity ofcolor by increasing the pitch, but the increased pitch causes a periodicstructure of the mat to appear within the field of view of the viewfinder, obstructing the view.

FIG. 1 shows a known diffusion plate having a pattern of microlenseshaving a maximum density arrangement. The microlenses are arranged at apitch of 16 μm, for example. Each microlens has 10 μm diameter and 1.6μm height.

FIGS. 2 through 10 show diagrams of various optical properties of thediffusion plate shown in FIG. 1.

FIGS. 2 through 4 show diffusion properties (diffraction patterns) ofthe plate by showing defocused images of point light sources havingwavelengths of 450 nm, 550 nm, and 650 nm, respectively. In FIGS. 2through 4, the diameters of the small circles (points) g correspond tothe intensity of diffracted light in the directions of the diameters,and the large circles h represent the F numbers of the bundles of raysincident on the diffusion plate, i.e., 2.0, 2.8, 4.0, 5.6, and 8.0, asviewed from the side of the outermost circle, respectively.

FIGS. 5 through 7 show the quantity of light at wavelengths of 450 nm,550 nm, and 650 nm, respectively (represented by the ordinate of thegraph) contained in the circles (encircled power depicted in FIGS. 2-4,respectively). The large circles have radii represented by the values onthe abscissa of the graph. The above assumes that the total quantity oflight transmitted through the diffusion plate, as shown in FIG. 1, is1.0.

FIGS. 8 through 10 show defocused image intensities of the line lightsources (line diffraction patterns in which the longitudinal line lightsource is represented by the solid lines and the lateral line lightsource is represented by the imaginary lines) at wavelengths of 450 nm,550 nm, and 650 nm, respectively. The abscissa represents the radii ofdiffraction of the diffraction patterns (diffusion angle of thediffusion plate), and the ordinate represents the relative intensity ofthe light, on the assumption that the peak intensity is 1.0,respectively.

FIG. 11 shows a structure of another known diffusion plate, having asquare arrangement of microlenses (microstructures) having 10 μmdiameter and 1.6 μm height at a uniform pitch of 16 μm.

FIGS. 12 through 20 show diagrams of various optical properties of adiffusion plate shown in FIG. 11.

FIGS. 12 through 14 show diffusion properties of the plate by showingdefocused images of a point light source having a wavelength of 450 nm,550 nm, and 650 nm, respectively. In FIGS. 2 through 4, the diameters ofthe small circles (points) g correspond to the intensity of diffractedlight in the directions of the diameters, and the large circles hrepresent the F numbers of the bundle of rays incident on the diffusionplate, i.e., 2.0, 2.8, 4.0, 5.6, and 8.0, as viewed from the side of theoutermost circle, respectively.

FIGS. 15 through 17 show the quantity of light (represented by theordinate) contained in the circles having radii represented by theabscissa, at the wavelengths of 450 nm, 550 nm, and 650 nm, on theassumption that the total quantity of light transmitted through thediffusion plate, as shown in FIG. 11, is 1.0.

FIGS. 18 through 20 show defocused image intensities of the line lightsources (longitudinal line light source represented by the solid linesand lateral line light source represented by the imaginary lines) at thewavelengths of 450 nm, 550 nm, and 650 nm, respectively. The abscissarepresents the radii of diffraction, and the ordinate represents therelative intensity of light, on the assumption that the peak intensityis 1.0.

As can be seen from the drawings discussed above, the knowntwo-dimensional periodic diffusion plates have a discrete diffusionproperty which largely depends on the wavelength, resulting in adeteriorated aberration property and irregularity in color.

SUMMARY OF THE INVENTION

The primary object of the present invention is to provide a diffusionplate, which is free from the drawbacks, which occur in the sanded mator the periodic mat and which can obtain a brighter field of viewwithout causing an off-axis aberration (multi-lined image) orirregularity in color.

Another object of the present invention is to provide a diffusion platein which the mat structure cannot be seen in the view finder.

The present invention is also addressed to a method for easilymanufacturing such a diffusion plate.

According to an aspect of the present invention, there is provided adiffusion plate including a plurality of superimposed basicmicrostructure patterns, wherein the microstructures are located in twodimensional periodic arrangements, and wherein lattice vectors,corresponding to the periodic arrangement of the microstructures, varyin accordance with the pattern.

According to another aspect of the present invention, there is provideda diffusion plate including members having diffusion surfaces on whichmicrostructures are formed in a predetermined arrangement, wherein themembers are superimposed, and wherein the microstructures of the membersare located in a two dimensional periodic arrangement and have latticevectors that vary depending on the members.

According to still another aspect of the present invention, there isprovided a focusing screen comprising a diffusion plate having adiffusion surface, and a Fresnel lens located adjacent to the diffusionplate, wherein a diffusion surface is provided on the surface of theFresnel lens opposite the diffusion plate.

A focusing screen includes a diffusion plate, having a plurality ofsuperimposed basic microstructure patterns, and a Fresnel lens locatedclose to the diffusion plate. The microstructures are located in twodimensional periodic arrangements. The lattice vectors, corresponding tothe periodic arrangement of the microstructures, vary in accordance withthe pattern. The microstructures of at least one of the basic patternscontain a random fluctuation of microstructures added to the referencetwo dimension period.

The invention is also directed to a method for preparing a master diefor a diffusion plate including steps of providing a substrate, with aphotosensitive material coated thereon, opposite a mask having a micropattern at a predetermined distance and, illuminating the mask frombehind with light, thereby projecting the micro pattern onto thephotosensitive material to be exposed. The steps further includedeveloping the photosensitive material to form the microstructures onthe substrate, wherein the substrate or the mask is rotated in apredetermined direction, within a plane, to at least two angularpositions of the mask, relative to the substrate, so that the exposuresare effected at each of the at least two relative angular positions toform a desired microstructure on the substrate.

The master die is for preparing a plastic molding die which can be usedto prepare a diffusion plate according to the present invention.

The transmission function f(x, y) of a diffusion plate, according to thepresent invention, is obtained by the multiplication of those f₁ (x, y),f₂ (x, y), f₃ (x, y), . . . of basic patterns which constitute thediffusion plate, as follows.

    f(x, y)=f.sub.1 (x, y)·f.sub.2 (x, y)·f.sub.3 (x, y) . . .

Accordingly, the diffusion property of the diffusion plate, i.e., theFourier transform of f(x, y) is obtained by the following equation;

    F(ω.sub.x, ω.sub.y)=F.sub.1 (ω.sub.x, ω.sub.y) ** F.sub.2 (ω.sub.x, ω.sub.y) ** F.sub.3 (ω.sub.x, ω.sub.y) **

wherein; F₁ (ω_(x), ω_(y)), F₂ (ω_(x), ω_(y)), F₃ (ω_(x), ω_(y)) . . .designate the Fourier transforms of the transmission functions f₁ (x,y), f₂ (x, y), f₃ (x, y), . . . of the basic patterns.

The diffusion properties of the respective basic patterns are discrete,but the resultant diffusion property of the diffusion plate, obtained asa result of the convolution integration, has a higher density than thoseof the basic patterns. This is due to the variation in the two-dimensionlattice vectors. As a result of this, the aberration property can beimproved.

Discreteness, by convolution integration, is improved as follows.

It is assumed that discreteness of the basic pattern A is represented byseven discrete spectra, as shown in FIG. 21. The basic pattern B isrepresented by seven discrete spectra which are obtained by turning thebasic pattern A by 90°, as shown in FIG. 22, respectively. The resultantdiscrete spectra C of the diffusion plate consisting of the basicpatterns A and B, are obtained by the convolution integration of FIGS.21 and 22, as shown in FIG. 23, in which there are 49 (=7×7) discretespectra.

It is apparent that the discreteness of the diffusion property, as shownin FIGS. 21 and 22, is improved by the arrangement illustrated in FIG.23.

The present disclosure relates to the subject matter contained inJapanese patent application Nos. 03-357495 (filed on Dec. 3, 1991),04-90392 (filed on Feb. 27, 1992), and 04-263853 (filed on Dec. 1, 1992)which are expressly incorporated herein by reference in their entirety.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described below in detail with reference to theaccompanying drawings, in which;

FIG. 1 is a schematic view of a structure of a known diffusion platehaving the highest possible density arrangement of micro lenses;

FIGS. 2, 3, and 4 are diagrams which show defocused images of a pointlight source by the diffusion plate, as shown in FIG. 1, at wavelengthsof 450 nm, 550 nm, and 650 nm, respectively;

FIGS. 5, 6, and 7 are diagrams showing distributions of light diffusedby the diffusion plate, as shown in FIG. 1, at wavelengths of 450 nm,550 nm, and 650 nm, respectively, wherein the abscissa represents theradii of circles and the ordinate represents the quantity of lightcontained in the circles;

FIGS. 8, 9, and 10 are diagrams showing defocused image intensities inthe vertical and horizontal directions, obtained by integrating thefunctions illustrated in FIGS. 5, 6, and 7, respectively;

FIG. 11 is a schematic view of a structure of another known diffusionplate having a square arrangement of microlenses;

FIGS. 12, 13, and 14 are diagrams which show defocused images of a pointlight source by the diffusion plate, as shown in FIG. 11, at wavelengthsof 450 nm, 550 nm, and 650 nm, respectively;

FIGS. 15, 16, and 17 are diagrams showing distributions of lightdiffused by a diffusion plate shown in FIG. 11, at wavelengths of 450nm, 550 nm, and 650 nm, respectively, wherein the abscissa representsthe radii of circles and the ordinate the quantity of light contained inthe circles;

FIGS. 18, 19, and 20 are diagrams showing defocused images in thevertical and horizontal directions, obtained by integrating thefunctions illustrated in 17, respectively;

FIG. 21 is a schematic diagram of a discrete diffusion property;

FIG. 22 is a schematic diagram of a discrete diffusion property which isobtained by rotating FIG. 21 by 90°;

FIG. 23 is a schematic diagram of a resultant diffusion property whichis obtained by a convolution integration of the diffusion properties, asshown in FIGS. 21, and 22, and is presented to explain the concept ofthe present invention;

FIGS. 24 and 25 are schematic diagrams of two basic patterns in a firstembodiment of the present invention;

FIG. 26 is a schematic view of a diffusion plate structure made by acombination of the arrangements, as shown in FIGS. 24 and 25, accordingto a first embodiment of the present invention;

FIGS. 27, 28, and 29 are diagrams which show defocused images of a pointlight source produced by the diffusion plate, as shown in FIG. 26, atwavelengths of 450 nm, 550 nm, and 650 nm, respectively;

FIGS. 30, 31, and 32 are diagrams showing distributions of lightdiffused by a diffusion plate, as shown in FIG. 26, at wavelengths of450 nm, 550 nm, and 650 nm, respectively, wherein the abscissarepresents the radii of circles and the ordinate represents the quantityof light contained in the circles;

FIGS. 33, 34, and 35 are diagrams showing defocused images in thevertical and horizontal directions, obtained by integrating thefunctions illustrated in FIGS. 30, 31, and 32, respectively;

FIG. 36 is a diagram showing the relationship between an F number of abundle of rays incident upon the diffusion plate, as shown in FIG. 1,and an orientation ratio of diffused light;

FIG. 37 is the diagram showing a relationship between an F number of abundle of rays incident upon a diffusion plate according to the firstembodiment of the present invention, as shown in FIG. 26, and anorientation ratio of diffused light;

FIGS. 38 and 39 are schematic diagrams of two basic patterns in a secondembodiment of the present invention;

FIG. 40 is a schematic view of a diffusion plate structure made by acombination of the arrangements, as shown in FIGS. 38 and 39, accordingto a second embodiment of the present invention;

FIGS. 41, 42, and 43 are diagrams which show defocused images of a pointlight source produced by the diffusion plate, as shown in FIG. 40, atwavelengths of 450 nm, 550 nm, and 650 nm, respectively;

FIGS. 44, 45, and 46 are diagrams showing distributions of lightdiffused by a diffusion plate the as shown in FIG. 40 at wavelengths of450 nm, 550 nm, and 650 nm, respectively, wherein the abscissarepresents the radii of circles and the ordinate represents the quantityof light contained in the circles;

FIGS. 47, 48, and 49 are diagrams showing defocused images in thevertical and horizontal directions, obtained by integrating the functionillustrated in FIGS. 44, 45 and 46, respectively;

FIGS. 50 and 51 are schematic diagrams of two different basic patternsin a third embodiment of the present invention;

FIG. 52 is a schematic view of a diffusion plate structure made by acombination of the arrangements, as shown in FIGS. 50 and 51, accordingto a third embodiment of the present invention;

FIGS. 53, 54, and 55 are diagrams which show defocused images of a pointimage provided by the diffusion plate, as shown in FIG. 52, atwavelengths of 450 nm, 550 nm, and 650 nm, respectively;

FIGS. 56, 57, and 58 are diagrams showing distributions of lightdiffused by the diffusion plate, as shown in FIG. 52 at wavelengths of450 nm, 550 nm, and 650 nm, respectively, wherein the abscissarepresents the radii of circles and the ordinate represents the quantityof light contained in the circles;

FIGS. 59, 60, and 61 are diagrams showing an unsharpness (defocus) ofimages in the vertical and horizontal directions, obtained byintegrating the functions illustrated in FIGS. 56, 57 and 58,respectively;

FIGS. 62 and 63 are schematic diagrams of two different basic patternsin a fourth embodiment of the present invention;

FIG. 64 is a schematic view of a diffusion plate structure made by acombination of the arrangements, as shown in FIGS. 62 and 63, accordingto a fourth embodiment of the present invention;

FIGS. 65, 66, and 67 are diagrams which show defocused images of a pointlight source produced by the diffusion plate fourth embodiment, as shownin FIG. 64, at wavelengths of 450 nm, 550 nm, and 650 nm, respectively;

FIGS. 68, 69, and 70 are diagrams showing distributions of lightdiffused by the diffusion plate, as shown in FIG. 64, at wavelengths of450 nm, 550 nm, and 650 nm, respectively, wherein the abscissarepresents the radii of circles and the ordinate represents the quantityof light contained in the circles;

FIGS. 71, 72, and 73 are diagrams showing a defocus of images in thevertical and horizontal directions, obtained by integrating thefunctions illustrated in FIGS. 68, 69, and 70, respectively;

FIGS. 74 and 75 are schematic diagrams of two different basic patternsin a fifth embodiment of the present invention;

FIG. 76 is a schematic view of a diffusion plate structure made by acombination of the arrangements, as shown in FIGS. 74 and 75, accordingto a fifth embodiment of the present invention;

FIGS. 77, 78, and 79 are diagrams which show defocused images of a pointlight source produced by the diffusion plate of the fifth embodiment, asshown in FIG. 76, with the light source having wavelengths of 450 nm,550 nm, and 650 nm, respectively;

FIGS. 80, 81, and 82 are diagrams showing distributions of lightdiffused by the diffusion plate, as shown in FIG. 76, at wavelengths of450 nm, 550 nm, and 650 nm, respectively, wherein the abscissarepresents the radii of circles and the ordinate represents the quantityof light contained in the circles;

FIGS. 83, 84, and 85 are diagrams showing a defocus of images in thevertical and horizontal directions, obtained by integrating thefunctions illustrated in FIGS. 80, 81 and 82, respectively;

FIG. 86 is a schematic view of a hexagonal pyramid structure having thehighest possible density arrangement, according to a sixth embodiment ofthe present invention;

FIG. 87 is a sectional view of a diffusion plate, according to a sixthembodiment of the present invention;

FIG. 88 is a plan view of a diffusion plate, according to a seventhembodiment of the present invention;

FIGS. 89, 90, and 91 are diagrams which show defocused images of a pointlight source by the diffusion plate of a seventh embodiment, as shown inFIG. 88, at wavelengths of 450 nm, 550 nm, and 650 nm, when φ is equalto -15° (φ=-15°), respectively;

FIGS. 92, 93, and 94 are diagrams showing distributions of lightdiffused by the diffusion plate, as shown in FIG. 88, at φ=-15° and atwavelengths of 450 nm, 550 nm, and 650 nm, respectively, wherein theabscissa represents the radii of circles and the ordinate represents thequantity of light contained in the circles;

FIGS. 95, 96, and 97 are diagrams showing a defocus of images in thevertical and horizontal directions, obtained by integrating FIGS. 92, 93and 94, at φ=-15° and at wavelengths of 450 nm, 550 nm, and 650 nm,respectively;

FIG. 98 is a schematic view of a Fresnel lens having a range of 36×24mm;

FIG. 99 is a schematic view of a moire pattern which is produced when adiffusion plate, as shown in FIG. 26 and a Fresnel lens, as shown inFIG. 98, are superimposed;

FIG. 100 is an explanatory view of a random positional fluctuation ofmicrostructures added to a two dimensional periodic arrangement;

FIGS. 101 and 102 are schematic diagrams of two different basic patternsin an eighth embodiment of the present invention;

FIG. 103 is a schematic view of a diffusion plate structure made by acombination of the arrangements, as shown in FIGS. 101 and 102,according to an eighth embodiment of the present invention;

FIGS. 104 and 105 are schematic diagrams of two different basic patternsin a ninth embodiment of the present invention;

FIG. 106 is a schematic view of a diffusion plate structure made by acombination of the arrangements, as shown in FIGS. 104 and 105,according to a ninth embodiment of the present invention;

FIG. 107 is a sectional view of a focusing screen according to a tenthembodiment of the present invention;

FIG. 108 is a sectional view of a focusing screen according to aneleventh embodiment of the present invention;

FIG. 109 is a sectional view of a substrate and a mask;

FIG. 110 is a plan view of a micro pattern formed on a mask shown inFIG. 109;

FIG. 111 is a plan view showing the relative rotation of a mask;

FIGS. 112a and 112b are a plan view of a projected micro pattern whichis produced when regular micro patterns are superimposed, and asectional view of an uneven micro relief formed by a projected micropattern, as shown in FIG. 112a, respectively; and,

FIG. 113 is a schematic view of the main components of a single lensreflex camera to which the present invention is applied, by way ofexample.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The illustrated embodiments, which will be discussed below, are appliedto a diffusion plate which is used as a focusing screen provided in, forexample, a view finder of a single lens reflex camera. In FIG. 113, abundle of rays, reflected from an object to be photographed, aretransmitted through a taking lens 101 and reflected by a quick returnmirror 102, before being converged onto a focusing screen 100, so that aphotographer can observe an image of the object through an eyepiece 103and a pentagonal prism 104 of a view finder.

EMBODIMENT 1

FIG. 24 shows a basic pattern of a diffusion plate, according to a firstembodiment, in which microlenses f (micro structures) of 1.2 μm heightand 10 μm diameter are arranged with the highest possible density at apitch of 16 μm. FIG. 25 shows another basic pattern of a diffusionplate, according to a first embodiment, in which microlenses f(microstructures) of 1.2 μm height are arranged in a manner having thehighest density possible. The microlenses are arranged having a pitch of16 μm, similar to FIG. 24, but there is a difference of 30° in theorientation between FIG. 24 and FIG. 25.

FIG. 26 shows a resultant pattern of a diffusion plate which is acombination of the basic patterns shown in FIGS. 24 and 25. Note thatthere is no periodicity in the resultant pattern of the superimposedbasic patterns. Although the resultant pattern may appear periodic, itis not.

FIGS. 27 through 35 show optical properties of the diffusion plate, asshown in FIG. 26. FIGS. 27 through 29 show diffusion properties, i.e.,an unsharpness (defocus) of a point light source at the wavelengths of450 nm, 550 nm, and 650 nm, respectively. In FIGS. 27 through 29, thediameters of the small circles (points) correspond to the intensity ofdiffracted light in the directions of the diameters thereof, and thelarge circles correspond to the F numbers of a bundle of rays incidentupon the diffusion plate. The F numbers are 2.0, 2.8, 4.0, 5.6, and 8.0,respectively, as viewed from the outermost circle.

FIGS. 30 through 32 show the quantity of light contained in the circlesat the wavelengths of 450 nm, 550 nm, and 650 nm, respectively. In FIGS.30 through 32, the abscissa represents the radii of the circles, and theordinate represents the quantity of light contained in the circles. Thegraphs assume that the total quantity of light transmitted through thediffusion plate, as shown in FIG. 26 is 1.0.

FIGS. 33 through 35 shows defocused image intensities of the line lightsources at wavelengths of 450 nm, 550 nm, and 650 nm. The solid linesdesignate the vertical line light sources and the imaginary lines thehorizontal line light sources, respectively. In FIGS. 33 through 35, theabscissae represent the radii of diffraction of the diffractionpatterns, and the ordinates the relative light intensity, respectively,on the assumption that the peak intensity is 1.0. In FIGS. 33 through35, since there is no difference in the distribution depending on thedirection of the line light sources, the solid lines and the imaginarylines overlap.

In comparison with the prior art shown in FIGS. 2 through 10, thediffusion property of the diffusion plate, as shown in FIGS. 27 through35, is indiscrete and less dependent on the wavelength, so that thediffusion property can be improved in the present invention.

FIGS. 36 and 37 show relationships between the F number of the incidentlight and the orientation ratio in the diffusion plates of the prior artand the present invention, as shown in FIG. 1 and FIG. 26, respectively.When the aperture is opened, the bundles of rays of 450 nm, 550 nm and650 nm are made incident upon the photographer's eye through theeyepiece lens at an equal ratio.

However, as the aperture is stopped-down, that is, as the F numberincreases, the orientation ratio changes the case of the periodicdiffusion plate. Accordingly, there is an irregularity in color of anobject image which is observed through the view finder. Conversely, inthe case of the diffusion plate, according to the present invention, ifthe F number changes, there will be little or no change in theorientation ratio. Accordingly, color irregularity does not occur, asshown in FIG. 37.

EMBODIMENT 2

FIG. 38 shows a basic pattern of a diffusion plate, according to asecond embodiment of the present invention, in which the micro lenses(microstructures) are 1.2 μm in height and 10 μm in diameter, and arearranged with the highest possible density at a pitch of 16 μm.

FIG. 39 shows another basic pattern of a diffusion plate, according to asecond embodiment, in which the micro lenses (microstructures) are 1.2μm in height and 10 μm in diameter, and are arranged with the highestpossible density at a pitch of 16 μm, similar to the basic pattern shownin FIG. 38. There is an angular phase difference between the basicpatterns shown in FIGS. 38 and 39. Namely, the basic pattern shown inFIG. 39 is obtained by turning the basic pattern of FIG. 38 by 21.78° tohave an orientation different from that of the basic pattern of FIG. 38.

FIG. 40 shows a resultant pattern of a diffusion plate which is acombination of the basic patterns shown in FIGS. 38 and 39. Theresultant pattern shown in FIG. 40, in which there is an angular phasedifference of 21.78°, is also periodical.

FIGS. 41 through 49 show various optical properties of the diffusionplate shown in FIG. 40.

FIGS. 41 through 43 show the state of diffusion, i.e., the defocusedimages of a point light source at wavelengths of 450 nm, 550 nm, and 650nm, respectively. In FIGS. 41 through 43, the diameters of the smallcircles (points) correspond to the intensity of the diffracted light inthose directions, and the diameters of the large circles correspond tothe F numbers of bundles of rays incident upon the diffusion plate. Inthe illustrated embodiment, the F numbers are 2.0, 2.8, 4.0, 5.6 and 8.0in this order as viewed from the outermost circle, respectively.

FIGS. 44 through 46 show the quantity of light at wavelengths of 450 nm,550 nm, and 650 nm, respectively. The abscissae designate the radii ofthe respective circles (angular coordinate) and the ordinates designatethe quantities of light contained in the respective circles on theassumption that the total quantity of light transmitted through thediffusion plate, as shown in FIG. 40, is 1.0, respectively.

FIGS. 47 through 49 show defocused image intensities of a line lightsource (longitudinal line light source shown at solid lines and lateralline light source shown at imaginary lines) at wavelengths of 450 nm,550 nm, and 650 nm, respectively. The abscissae designate the radii ofdiffraction of the diffraction patterns (i.e., angular coordinates ofdiffraction) and the ordinates show the relative intensity of light onthe assumption that the peak intensity is 1.0, respectively.

Although the resultant pattern is discrete due to the periodicity of thebasic patterns to be superimposed, the optical properties of thediffusion plate, according to the illustrated embodiment, are highlyimproved in comparison with the diffusion plate shown in FIGS. 2 through10. Furthermore, the structure of the diffusion plate is not as visibleas in the conventional diffusion plate in which the pitch of themicrostructures are merely increased.

EMBODIMENT 3

FIG. 50 shows a basic pattern of a diffusion plate according to a thirdembodiment of the present invention, in which the micro lenses(microstructures) of 1.2 μm height and 10 μm diameter are arranged withthe highest possible density at a pitch of 16 μm.

FIG. 51 shows another basic pattern of a diffusion plate according to athird embodiment, in which the micro lenses (microstructures) of 1.2 μmheight and 6.67 μm diameter are arranged with the highest possibledensity at a pitch of 10.67 μm. The two basic patterns have the sameorientation.

FIG. 52 shows a resultant pattern of a diffusion plate which is acombination of the basic patterns shown in FIGS. 50 and 51. Theresultant pattern shown in FIG. 52 is also periodic.

FIGS. 53 through 61 show various optical properties of the diffusionplate shown in FIG. 52.

FIGS. 53 through 55 show the state of diffusion, i.e., the defocusedimages of a point light source at wavelengths of 450 nm, 550 nm, and 650nm, respectively. In FIGS. 53 through 55, the diameters of the smallcircles (points) correspond to the intensity of the diffracted light,and the diameters of the large circles correspond to the F numbers ofbundles of rays incident upon the diffusion plate. In the illustratedembodiment, the F numbers are 2.0, 2.8, 4.0, 5.6 and 8.0, in this order,as viewed from the outermost circle, respectively.

FIGS. 56 through 58 show the quantity of light at wavelengths of 450 nm,550 nm, and 650 nm, respectively. The abscissae designate the radii ofthe respective circles and the ordinates designate the quantities oflight contained in the respective circles on the assumption that thetotal quantity of light transmitted through the diffusion plate, asshown in FIG. 52, is 1.0, respectively.

FIGS. 59 through 61 show defocused image intensities of a line lightsource (longitudinal line light source shown at solid lines and lateralline light source shown at imaginary lines) at wavelengths of 450 nm,550 nm, and 650 nm, respectively. The abscissae designate the radii ofdiffraction of the diffraction patterns, and the ordinates the relativeintensity of light on designate the assumption that the peak intensityis 1.0, respectively.

Although the resultant pattern is discrete due to the periodicity of thebasic patterns to be superimposed, the optical properties of thediffusion plate, according to the illustrated embodiment, are highlyimproved in comparison with the diffusion plate shown in FIGS. 2 through10. Furthermore, the structure of the diffusion plate is not as visibleas in the conventional diffusion plate in which the pitch of themicrostructures is merely increased.

In the third embodiment, although the diffusion plate is made of twobasic patterns having the same orientation and different pitches, it ispossible to superimpose the basic patterns with an angular phasedifference therebetween, similar to the first or second embodiment.Namely one of the basic patterns may be rotated by a predetermined anglewith respect to the other basic pattern.

EMBODIMENT 4

FIG. 62 shows a basic pattern of a diffusion plate, according to afourth embodiment of the present invention, in which the micro lenses(microstructures) of 1.2 μm height and 10 μm diameter are distributed ata pitch of 16 μm in a square arrangement.

FIG. 63 shows another basic pattern of a diffusion plate according to afourth embodiment, in which the micro lenses (microstructures) of 1.2 μmheight and 10 μm diameter are distributed at a pitch of 16 μm in asquare arrangement. There is an angular phase difference of 45° betweenthe basic patterns. Namely, the orientation of the first basic pattern,as shown in FIG. 62, is different from that of the second basic pattern,as shown in FIG. 63, by 45°.

FIG. 64 shows a resultant pattern of a diffusion plate which is acombination of the basic patterns shown in FIGS. 62 and 63. Theresultant pattern shown in FIG. 64 has no periodicity. Although theresultant pattern may appear periodic, it is not.

FIGS. 65 through 73 show various optical properties of the diffusionplate shown in FIG. 64.

FIGS. 65 through 67 show the state of diffusion, i.e., the defocusedimages of a point light source at wavelengths of 450 nm, 550 nm, and 650nm, respectively. In FIGS. 65 through 67, the diameters of the smallcircles (points) correspond to the intensity of the diffracted light,and the diameters of the large circles correspond to the F numbers ofbundles of rays incident upon the diffusion plate. In the illustratedembodiment, the F numbers are 2.0, 2.8, 4.0, 5.6 and 8.0, in this order,as viewed from the outermost circle, respectively.

FIGS. 68 through 70 show the quantity of light at wavelengths of 450 nm,550 nm, and 650 nm, respectively. The abscissae designate the radii ofthe respective circles and the ordinates designate the quantities oflight contained in the respective circles on the assumption that thetotal quantity of light transmitted through the diffusion plate, asshown in FIG. 64, is 1.0, respectively.

FIGS. 71 through 73 show defocused image intensities of a line lightsource (longitudinal line light source shown at solid lines and lateralline light source shown at imaginary lines) at wavelengths of 450 nm,550 nm, and 650 nm, respectively. The abscissae designate the radii ofdiffraction of the diffraction patterns and the ordinates designate therelative intensity of light on the assumption that the peak intensity is1.0, respectively.

Since there is no difference in distribution in the direction of theline light source, the solid lines and the imaginary lines overlap.

EMBODIMENT 5

FIG. 74 shows a basic pattern of a diffusion plate according to a fifthembodiment of the present invention, in which the micro lenses(microstructures), of 1.2 μm height and 10 μm diameter, are distributedat a pitch of 16 μm in the highest possible density arrangement. Thelattice vector is represented by p=(16, 0)(μm) and q=(8, 8·3^(1/2))(μm).Unless otherwise specified, the components of the lattice vectors aredefined in terms of the Cartesian Coordinate System.

FIG. 75 shows another basic pattern of a diffusion plate according to afifth embodiment, in which the micro lenses (microstructures) of 1.2 μmheight and 10 μm diameter are distributed at a pitch of 16 μm in asquare arrangement. There is an angular phase difference of 45° betweenthe basic patterns. Namely, the orientation of the first basic patternshown in FIG. 75 is different from that of the second basic pattern, asshown in FIG. 74, by 45°. The lattice vector is represented byp=(8·2^(1/2), 8·2^(1/2))(μm) and q=(-8·2^(1/2), 8·2^(1/2))(μm).

FIG. 76 shows a resultant pattern of a diffusion plate which is acombination of the basic patterns shown in FIGS. 74 and 75. Theresultant pattern, as shown in FIG. 76 has no periodicity. Although theresultant pattern may appear periodic, it is not.

FIGS. 77 through 85 show various optical properties of the diffusionplate shown in FIG. 76.

FIGS. 77 through 79 show the state of diffusion, i.e., the defocusedimage of a point light source at wavelengths of 450 nm, 550 nm, and 650nm, respectively. In FIGS. 77 through 79, the diameters of the smallcircles (points) correspond to the intensity of the diffracted light,and the diameters of the large circles correspond to the F numbers ofbundles of rays incident upon the diffusion plate. In the illustratedembodiment, the F numbers are 2.0, 2.8, 4.0, 5.6 and 8.0, in this order,as viewed from the outermost circle, respectively.

FIGS. 80 through 82 show the quantity of light at wavelengths of 450 nm,550 nm, and 650 nm, respectively. The abscissae designate the radii ofthe respective circles and the ordinates designate the quantities oflight contained in the respective circles on the assumption that thetotal quantity of light transmitted through the diffusion plate, asshown in FIG. 76 is 1.0, respectively.

FIGS. 83 through 85 show defocused image intensities of a line lightsource (longitudinal line light source shown at solid lines and lateralline light source shown at imaginary lines) at wavelengths of 450 nm,550 nm, and 650 nm, respectively. The abscissae designate the radii ofdiffraction of the diffraction patterns and the ordinates the relativeintensity of light on the assumption that the peak intensity is 1.0,respectively.

In the above-mentioned embodiments, the microstructures f are all microlenses, each having a circular shape in the direction normal to thebasic patterns. The micro lenses which are most preferably circular inshape can be replaced with, for example, hexagonal pyramid lenses f'which are closely located side by side, as shown in FIG. 86. A pluralityof basic patterns, each having the hexagonal pyramid lenses f' aresuperimposed with a predetermined angular phase difference. In thiscase, the discreteness of the diffusion property is improved by aconvolute integration and no color irregularity occurs.

The factor (envelope) which determines the relative intensity of thediffusion property of the discrete spectra of the periodic basicpatterns is a Fourier transform G(ω_(x), ω_(y)) of the transmissionfunction g(x, y) of the microstructures. Accordingly, if transmissionfunction g(x, y) is of rotation symmetry, the Fourier transform G(ω_(x),ω_(y)) is also of rotation symmetry. The envelope of the diffusionproperty of the diffusion plate which is made of the basic patterns isalso of rotational symmetry, since it is a convolute integration of afunction of rotation symmetry. Consequently, the shape of themicrostructures as viewed from the direction normal to the basicpatterns is preferably circular to improve the diffusioncharacteristics.

EMBODIMENT 6

FIG. 87 shows a sectional shape of a diffusion plate according to asixth embodiment of the present invention. The diffusion plate can beproduced from a single plate by a manufacturing method as will bediscussed hereinafter, or from a plurality of plates having thediffusion surfaces with the basic patterns closely opposed to eachother, as shown in FIG. 87. In FIG. 87, the diffusion plates 1 and 2,having the basic patterns, are opposed, so that the diffusion surfaces15 and 25 thereof face each other. In the arrangement illustrated inFIG. 87, there are concave microstructures 3 which can be replaced withconvex microstructures.

EMBODIMENT 7

FIG. 88 shows a plan view of a diffusion plate according to a seventhembodiment of the present invention. In the arrangement shown in FIG.88, there are two diffusion plates 4 and 5 having the same basicpatterns at a predetermined angle φ (angular phase difference)therebetween. The diffusion plates 4 and 5 are relatively rotatable tovary the angle φ between -15° and +15°. When φ=-15°, the basic patternsare oriented in the same direction. Numeral 6 designates the field ofview of the finder.

The basic patterns are the same as that shown in FIG. 24 in which themicro lenses (microstructures), of 1.2 μm height and 10 μm diameter, aredistributed at a pitch of 16 μm in the highest possible densityarrangement. FIGS. 89 through 97 show various optical properties of thediffusion plate according to the seventh embodiment when φ=-15°.

FIGS. 89 through 91 show the state of diffusion, i.e., the defocusedimages of a point light source at wavelengths of 450 nm, 550 nm, and 650nm, respectively. In FIGS. 89 through 91, the diameters of the smallcircles (points) correspond to the intensity of the diffracted light inthose directions, and the diameters of the large circles correspond tothe F numbers of bundles of rays incident upon the diffusion plate. Inthe illustrated embodiment, the F numbers are 2.0, 2.8, 4.0, 5.6 and8.0, in this order as viewed from the outermost circle, respectively.

FIGS. 92 through 94 show the quantity of light at wavelengths of 450 nm,550 nm, and 650 nm, respectively. The abscissae designate the radii ofthe respective circles and the ordinates designate the quantities oflight contained in the respective circles on the assumption that thetotal quantity of light transmitted through the diffusion plate, asshown in FIG. 26, is 1.0.

FIGS. 95 through 97 show defocused image intensities of a line lightsource (longitudinal line light source shown at solid lines and lateralline light source shown at imaginary lines) at wavelengths of 450 nm,550 nm, and 650 nm, respectively. The abscissae designate the radii ofdiffraction of the diffraction patterns, and the ordinates designate therelative intensity of light on the assumption that the peak intensity is1.0.

It is possible to provide diffusion plates having various opticalproperties, including e.g. those of the diffusion plate shown in FIG. 40by making φ=6.78°, and those of the diffusion plate shown in FIG. 26 bymaking φ=15°. A photographer can optionally select, for example, φ=15°for improved diffusion characteristics, φ=6.78° for a smoother matstructure, or φ=-15° for an intentional off-axis aberration to confirm afocus state.

As can be understood from the above-discussion, it is possible toprepare a light diffusion plate having improved diffusioncharacteristics and less irregularity of color by superimposing basicpatterns having microstructures f in a two dimensional periodicarrangement.

The present invention also is directed to the elimination of moirefringes. Specifically, if the diffusion plate is used as a focusingscreen 100 in a single lens reflex camera, there is a possibility thatmoire fringes will occur between the resultant pattern and the patternof the Fresnel lens which is located in front of the focusing screen asa condenser lens, since the resultant pattern has periodicity even whenthe non-periodic patterns are superimposed. The moire fringes have anadverse influence on the determination of composition or the visualconfirmation of defocus.

FIG. 98 shows a Fresnel lens F within an area of 36 mm×24 mm. FIG. 99shows moire fringes which appear when the diffusion plate having thebasic patterns shown in FIG. 26 and the Fresnel lens F are superimposed.

To solve the problem mentioned above, microstructures of at least one ofthe superimposed basic patterns are re-arranged by randomly fluctuatingthe positions along two dimensions so that a non-periodic superimposedpattern is formed to prevent the moire fringes.

The coordinates of the lattice points in the two dimensional periodicarrangement are represented by r_(mn) =mp+nq, wherein p and q designatethe lattice vectors, as shown in FIG. 100. If the random positionalfluctuation Δr_(mn) is added to this coordinate, the coordinate g(r) ofthe microstructures is given by (r_(mn) +Δr_(mn)). The transmissionfunction f(r) of the basic pattern is mathematically obtained by thefollowing equation. ##EQU3##

For instance, in case of the highest density arrangement in which themicrostructures are located at the center and apexes of a regularhexagon, |p|=|q|, and |p|·|q|=|p|·|q|/2, and in case of the squarearrangement which the micro structures are located at the apexes of asquare, |p|=|q|, and p·q=0.

When the fluctuation increases, the moire fringes disappear. Theincreased fluctuation however deteriorates the quality of the focusingscreen. Namely, the screen surface becomes undesirably rough.

According to the inventors' experiments, for the diffusion plate havingthe basic patterns superimposed in the highest possible density with anangular phase difference of 90°, wherein the length of the latticevectors was 16 μm, the relationship between the standard deviation σ(μm) of fluctuation, the inconspicuousness of the moire fringes, and thesurface smoothness of the screen is as shown in Table 1 below. In Table1, "x" designates that the moire fringes are conspicuous and the screensurface is rough, "Δ" that the moire fringes are little conspicuous andthe screen surface is fairly smooth, and "∘" that the moire fringes arecompletely inconspicuous and the screen surface is smooth, respectively.

                  TABLE 1                                                         ______________________________________                                        standard deviation                                                            of fluctuation σ (μm)                                                               0.0   0.7     1.4 2.1   2.8 3.5                                ______________________________________                                        inconspicuousness                                                                            X     Δ ◯                                                                     ◯                                                                       ◯                                                                     ◯                      smoothness     ◯                                                                       ◯                                                                         ◯                                                                     Δ                                                                             Δ                                                                           X                                  ______________________________________                                    

As can be seen from the foregoing, there are limits in the extend towhich random positioned fluctuations will produce a diffusion platehaving the appropriate optical properties. Namely, the standarddeviation σ of the fluctuation preferably satisfies the followingrelationship with respect to the mean value ρ of the lengths of thelattice vectors. ##EQU4## wherein Δr_(mk), nk designates the fluctuationvector of the (m_(k), n_(k))-th microstructure contained in the k-thbasic pattern, p_(k), q_(k) the lattice vector of the two dimensionperiod of the k-th basic pattern, ##EQU5## the sum of the vectors of thebasic patterns to be superimposed, and ##EQU6## the sum of the vectorsof the microstructures contained in the statistical area for the k-thbasic pattern, respectively.

EMBODIMENT 8

FIG. 101 shows a basic pattern of a diffusion plate according to aneighth embodiment, in which the micro lenses (microstructures) of 1.2 μmheight and 10 μm diameter are arranged with the highest possible densityat a pitch of 16 μm. The lattice vector is represented by p=(16, 0)(μm)and q=(8, 8·3^(1/2))(μm), and the standard deviation p of thefluctuation is ρ=1.4 (μm).

FIG. 102 shows another basic pattern of a diffusion plate according toan eighth embodiment, in which the micro lenses (microstructures) of 1.2μm height and 10 μm diameter are arranged with the highest possibledensity at a pitch of 16 μm. The lattice vector is represented by p=(0,16)(μm) and q=(-8·3^(1/2), 8)(μm), and the standard deviation ρ of thefluctuation is ρ=1.4 (μm). The basic pattern, as shown in FIG. 102, isobtained by turning the basic pattern, as shown in FIG. 101, by 90°.

FIG. 103 shows a resultant pattern of a diffusion plate which is acombination of the basic patterns shown in FIGS. 101 and 102. In theresultant pattern, σ/ρ=0.088.

EMBODIMENT 9

FIG. 104 shows a basic pattern of a diffusion plate according to anninth embodiment, in which the micro lenses (micro structures) of 1.2 μmheight and 10 μm diameter are arranged with the highest possible densityat a pitch of 16 μm. The lattice vector is represented by p=(16, 0) (μm)and q=(8, 8·3^(1/2))(μm), and the standard deviation ρ of thefluctuation is ρ=0 (μm).

FIG. 105 shows another basic pattern of a diffusion plate according to aninth embodiment, in which the micro lenses (microstructures) of 1.2 μmheight and 10 μm diameter are arranged with the highest possible densityat a pitch of 16 μm. The lattice vector is represented by p=(0, 16)(μm)and q=(-8·3^(1/2), 8)(μm), and the standard deviation p of thefluctuation is ρ=1.4 (μm), similar to the basic pattern shown in FIG.102.

FIG. 106 shows a resultant pattern of a diffusion plate which is acombination of the basic patterns shown in FIGS. 104 and 105. In theresultant patterns σ/ρ=0.044.

As can be understood from the above discussion, if a fluctuation isadded to at least one of the basic patterns to be superimposed, theresultant pattern obtained by superimposing the basic patterns isirregular, so that little or no moire fringes occur between theresultant pattern and the pattern of the Fresnel lens. Better diffusioncharacteristics and less color irregularity can be expected from theresultant pattern thus obtained in comparison with the diffusion platewhich is obtained by superimposing the basic patterns having nofluctuation added thereto.

EMBODIMENT 10

FIG. 107 shows a cross sectional view of a focusing screen of adiffusion plate according to a tenth embodiment, in which a diffusionsurface 7 is formed on one of opposite surfaces of a plate and a Fresnellens 8 is formed on the other surface of the same plate, respectively.

EMBODIMENT 11

FIG. 108 shows a cross sectional view of a focusing screen of adiffusion plate according to an eleventh embodiment, in which thediffusion surfaces, having basic patterns 9 and 10, are provided onopposed diffusion plates 11 and 12. The Fresnel lens 13 is formed on thesurface of the diffusion plate 12 opposite to the diffusion surface.

EMBODIMENT 12

The following discussion will be directed to a method for manufacturinga master die for a diffusion plate according to the present invention.

In FIG. 109, a substrate 14, which is made of, for example, glass, isprovided with a positive photoresist film 15 of a uniform thickness(e.g., 2˜3 μm thickness) coated thereon by a spin-coat method, forexample, as a photosensitive material.

An exposing glass mask 16, which is used to form a predetermined patternexposed onto the positive photoresist film 15 is provided with a dotpattern 17 of chrome in the shape of micro projections, as shown in FIG.110. The micro pattern 17 is made of micro lenses having a diameter of10˜20 μm and spaced from one another at a pitch of 15˜20 μm.

The mask 16 is spaced from the substrate 14 at a distance Δt (e.g.,about 75 μm). Thereafter, the mask 16 is illuminated with UV (ultraviolet) light 18 from behind the micro pattern 17 of the mask 16 for apredetermined time to project and expose the micro pattern 17 onto thesurface of the positive photoresist film 15.

After that, as shown in FIG. 111, the mask 16 is rotated in apredetermined direction by an angle θ (e.g., θ=5°) about an axis Lperpendicular to the sheet of the drawing within the same plane, and theexposure is then effected again.

After these operations are finished, the substrate 14 is subject to thedevelopment process, so that the uneven relief pattern 19 (microprojections and indentations), as shown in FIG. 112a, is formed on thepositive photoresist film 15.

The relief pattern 19, thus obtained, is not regular and includesdifferent heights and shapes of micro projections and depressions.Namely, highest portions X are formed by the projected and overlappeddots 20 which constitute the micro pattern 19, intermediate portions Yare formed by the projected dots 20 without being overlapped, and lowestportions Z are formed by the portions other than the dots 20.

Since the distance Δt between the positive photoresist film 15 and themask 16 is 75 μm, so that the scattered UV light 18 can reach the micropattern 17, the portions other than the dots 20 are not linear and aresmoothly curved, so that the whole micro pattern 19 defines a smoothlycurved surface.

The substrate 14 can be thus manufactured by copying the master die. Theelectroforming die is prepared by the substrate 14, and the pattern canbe transferred to an optical resin material using an injection molding,for example, to manufacture a light diffusion plate having an improveddiffusibility, less granularity and an improved diffusioncharacteristic.

The mask 16 is usually formed by a photolithography process which is perse known. A regular pattern, as shown in FIG. 1, or a pattern having apositional fluctuation, as shown in FIG. 101, is drawn by a photoplotter or the like to prepare a reticle. The reticle is then projectedby reduced magnification and discontinuously moved to be repeatedlyexposed, so that a desired size of mask can be prepared. The size andthe pitch, etc., of the micro patterns 19 can be optionally selecteddepending on the required properties of the diffusion plate. The randompositional deviation can be also controlled in pseudo-random numbers bya computer.

In the illustrated embodiment, the diffusion plate is formed bysuperimposing the two dimensional periodic patterns of microstructuresor superimposing a random positional fluctuation of microstructures toat least one of the two dimensional periodic patterns, as mentionedabove. Nevertheless, the diffusion plate does not necessarily haveuniform periodicity as a whole. Namely, it is possible to use adiffusion plate which has different basic patterns at the center portionand peripheral portion thereof.

Although the master die is prepared by multi-exposure while relativelyrotating the mask, in the illustrated embodiment, it is possible toprepare a master die by one exposure using a mask on which theoverlapped pattern is drawn in advance.

The number of the basic patterns to be superimposed is not limited totwo and can be more than two in the present invention.

As can be seen from the above discussion, according to the presentinvention, a light diffusion plate is obtained whose diffusion propertycan be controlled so as not to produce a rough surface visual affectwhen used as a focusing screen. The light diffusion plate thus obtainedis substantially free from an off-axis aberration and an irregularity incolor. No moire fringe occurs between the Fresnel lens and the diffusionplate of the present invention.

Furthermore, unlike the periodic diffusion plate in which even a slightconstitutional fault of the microstructure is conspicuous, a diffusionplate of the present invention prevents such a fault from beingconspicuous, thus resulting in an increased yield of good products.

We claim:
 1. A method for manufacturing a master die for a diffusionplate, comprising the steps of:providing a substrate having aphotoresist; providing a first mask having a first pattern; illuminatingthe photoresist through the mask, forming an image of the first patternin the photoresist; providing a second mask having a second pattern, thesecond pattern having an arrangement identical to, and a pitch smallerthan, the first pattern; again illuminating the photoresist through thesecond mask, to form an image of the second pattern in the photoresistto form a combined pattern in said photoresist.
 2. The method ofmanufacturing a master die according to claim 1, wherein the combinedpattern is periodic.
 3. The method of manufacturing a master dieaccording to claim 1, wherein the combined pattern is non-periodic. 4.The method of manufacturing a master die according to claim 1, whereinthe first and second patterns include a plurality of circles arrangedwith a maximum density.
 5. The method of manufacturing a master dieaccording to claim 1, wherein said first pattern has a positionalfluctuation.
 6. The method of manufacturing a master die according toclaim 1, wherein said first pattern includes a plurality of hexagonalshapes having a maximum density.
 7. The method of manufacturing a masterdie according to claim 1, wherein the combined pattern is a smoothcurved surface having an irregular curvature.